The Vizards are the antithesis to the Arrancar as Shinigami who have obtained Hollow powers. Having begun as Shinigami, the Vizards appear entirely Human, and barring those times where they are using their powers are indistinguishable from ordinary Shinigami. To gain access to their Hollow powers, a Vizard dons a Hollow mask. This allows them to augment their Shinigami abilities with Hollow powers, as well as giving them access to some of the generic Hollow abilities. Their spiritual pressure likewise changes into a mix of Hollow and Shinigami. This process of donning one's Hollow mask is referred to as Hollowfication.
It is unclear what the Vizards are after. The current members of the Vizards have been shown to be former captain or lieutenant-class Shinigami. They are considered criminals by the Soul Society for committing the ultimate taboo of acquiring Hollow powers; their status as former leaders of Soul Society also marks them as traitors. The Vizards are also rejected by Hollows, due to their former status as Shinigami. It is because of this that it can be presumed that the only side the Vizards are on is their own. Regardless, they seem to be valuable allies-to-be in the battle against Sōsuke Aizen's army of Arrancar, as Kisuke Urahara mentioned. The Vizards have not shown any sort of hierarchy, seeming to prefer a loose-knit organization.